Fiber optic cable for use in harsh environments

ABSTRACT

Fiber optic cables suitable for use in harsh environments such as down hole oil and gas well applications and methods for fabricating the same have been provided. In one embodiment, an optic cable suitable for down hole oil field applications comprises one or more optical fibers disposed in an inner tube and a corrosion resistant metal outer tube disposed over the inner tube, where the inner and outer tubes make intermittent contact. In another embodiment, an optic cable suitable for down hole oil field applications comprise one or more optical fibers disposed in a polymer tube having fins extending therefrom.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to fiber optic cables foruse in harsh environments such as down hole gas and oil wellapplications.

2. Background of the Related Art

With advancements in the area of fiber optic sensors for use in harshenvironments, there is an increasing need for fiber optic cablescompatible with the harsh environmental conditions present in down holeoil and gas well applications. For example, fiber optic cables utilizedin down hole sensing applications must be able to operate reliably inconditions that may include temperatures in excess of 300 degreesCelsius, static pressures in excess of 20,000 pounds per square inch(psi), vibration, corrosive chemistry and the presence of high partialpressures of hydrogen. As the sensors utilized in down hole applicationsmay be positioned at depths up to and exceeding 20,000 feet, the fiberoptic cable coupled thereto must be designed to support the opticalfiber contained therein without subjecting the optical fiber to thestrain associated with the weight of a long fiber suspended in avertical orientation within a well without disadvantageously effectingthe fiber's optical performance.

FIG. 7 depicts one example of a conventional fiber optic cable 700suitable for use in harsh environments such as down hole oil and gaswell applications. A similarly suitable cable is described in U.S. Pat.No. 6,404,961, issued Jun. 11, 2002 to Bonja, et al., which is herebyincorporated by reference in its entirety. Suitable cables are alsoavailable from Weatherford, Inc., located in Houston, Tex. The fiberoptic cable 700, shown in FIG. 7, includes a fiber in metal tube (FIMT)core 702 surrounded by an outer protective sleeve 704. The FIMT core 702includes an inner tube 706 surrounding one or more optical fibers 708.Three optical fibers 708 are shown disposed within the inner tube 706 inthe embodiment of FIG. 7. A filler material 710 is disposed in the innertube 706 to fill the void spaces not occupied by the optical fibers 708.The filler material 710 may also include a hydrogen absorbing/scavengingmaterial to minimize the effects of hydrogen on the optical performanceof the fiber 708. At least one of the inner or outer surface of theinner tube 706 is coated or plated with a low hydrogen permeabilitymaterial 716 to minimize hydrogen diffusion into the area which in theoptical fibers 708 are disposed.

The outer protective sleeve 704 includes a buffer material 712 and anouter tube 714. The buffer material 712 provides a mechanical linkbetween the inner tube 706 and the outer tube 714 to prevent the innertube 706 from sliding within the outer tube 714. Additionally, thebuffer material 712 keeps the inner tube 706 generally centered withinthe outer tube 714 and protects the inner tube 706 and coatings formedthereon from damage due to vibrating against the outer tube 714.

Although this cable design has shown itself to be a robust and reliablemeans for providing transmission of optical signals in harshenvironments such as oil and gas wells, the cable is one of the highercost contributors to the overall cost of down hole sensing systems.Additionally, as the diameter of the cable is typically aboutone-quarter inch, the length of cable that may be transported on a spoolusing conventional means is limited to about 20,000 feet of cable. Thus,in many down hole well applications, only a single sensor may be coupledto a length of cable coming off a single spool, as the residual lengthof cable on the spool is not long enough for another down holeapplication without splicing on an addition cable segment. As cost isprimary advantage of conventional metal conductor sensing systems overoptical systems, a more cost effective optic cable suitable for downhole oil well service is highly desirable.

Therefore, there is a need for an improved fiber optic cable for use inharsh environments.

SUMMARY OF THE INVENTION

Fiber optic cables suitable for use in harsh environments such as downhole oil and gas well applications and methods for fabricating the sameare provided. In one embodiment, an optic cable suitable for down holeoil field applications comprises one or more optical fibers disposed inan inner tube. A corrosion resistant metal outer tube is disposed overthe inner tube, where the inner and outer tubes make intermittentcontact. In another embodiment, an optic cable suitable for down holeoil field applications comprise one or more optical fibers disposed in apolymer tube having fins extending therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

A more particular description of the invention, briefly summarizedabove, may be had by reference to the embodiments thereof that areillustrated in the appended drawings. It is to be noted, however, thatthe appended drawings illustrate only typical embodiments of thisinvention and are therefore not to be considered limiting of its scope,for the invention may admit to other equally effective embodiments.

FIG. 1 is a cross sectional view of one embodiment of a fiber opticcable suitable for use in down hole oil and gas well applications;

FIG. 2 is a partial sectional side view of the optic cable of FIG. 1;

FIGS. 3A–E are cross sectional views of alternative embodiments of afiber optic cable suitable for use in down hole oil and gas wellapplications;

FIG. 4 is a cross sectional view of another embodiment of a fiber opticcable suitable for use in down hole oil and gas well applications;

FIG. 5 flow diagram of one embodiment of a method for fabriacting afiber optic cable suitable for use in down hole oil and gas wellapplications.

FIG. 6 is a simplified schematic of one embodiment of a fiber opticcable assembly line; and

FIG. 7 depicts one example of a conventional fiber optic cable suitablefor use in down hole oil and gas well applications.

To facilitate understanding, identical reference numerals have beenused, wherever possible, to designate identical elements that are commonto the figures.

DETAILED DESCRIPTION

FIG. 1 is one embodiment of a fiber optic cable 100 suitable for use indown hole oil and gas well applications. The cable 100 comprises a fiberin metal tube (FIMT) core 102 disposed in a protective outer tube 104.The FIMT 102 comprises an inner tube 106 surrounding one or more opticalfibers 108, three of which are shown in the embodiment depicted in FIG.1.

The inner tube 106 is fabricated from a corrosion resistant material.Examples of suitable corrosion resistant metal alloys include, but arenot limited to, 304 stainless steel, 316 stainless steel, INCONEL® 625and INCOLOY® 825, among others. Examples of suitable plastics include,but are not limited to fluoropolymers, ethylene-chlorotrifluoroethylene,fluoroethylenepropylene, polyvinylidene fluoride, polyvinylchoride,HALAR®, TEFLON® and TEFZEL®, among others. The diameter of the innertube 106 may be in the range of about 1.1 to about 2.6 mm, and in anexemplary embodiment of the invention is about 2.4 mm. Although theinner tube 106 is described as being about 1.1 to about 2.6 mm indiameter, the diameter of the inner tube 106 may vary, depending uponthe materials used and the number of optical fibers 108 to be placed inthe inner tube 106.

In one embodiment, the inner tube 106 has a wall thickness suitable fora seam welding process utilized to fabricate the tube from a coil ofmetal strip. For example, the wall thickness of the 304 stainless steelinner tube 106 may be about 0.2 mm to facilitate a continuous laser weldduring a tube forming process. In another embodiment, the inner tube 106has a wall thickness suitable for fabrication by plastic extrusion.

An optional plated barrier coating 110 may be disposed on at least oneof the inner or outer surfaces of the inner tube wall. The barriercoating 110 may be coated, plated or otherwise adhered to the inner tube106 and may be comprised of a low hydrogen permeability material, suchas tin, gold, carbon, or other suitable material. The thickness of thebarrier coating 110 is selected to slow the diffusion of hydrogen intothe center of the inner tube 106 driven by a high partial pressurehydrogen environment present in some wells. Depending upon the barriercoating material, the coating thickness may be in the range of about 0.1to about 30 microns or thicker. For example, a carbon barrier coating110 may have a thickness of about 0.1 microns, while a tin barriercoating 110 may have a thickness of approximately 13 microns. In oneembodiment, the barrier coating 110 includes a nickel seed layerdisposed on the tube surface that provides an adhesion layer for anouter layer of low hydrogen permeability material. In applications wherehigh partial pressures of hydrogen are not expected, the barrier coating110 may be omitted.

In one embodiment, a protective outer coating 112 is disposed over thebarrier coating 110. The outer coating 112 is a protective layer ofhard, scratch resistant material, such as nickel or a polymer such aspolyamide, among others, that substantially prevents the barrier coating110 from damage from contact with the outer tube 104. The outer coating112 may have a thickness in the range of about 0.5 to about 15 microns,depending on the selected material.

A filler material 114 is disposed in the inner tube 106 andsubstantially fills the void spaces within the inner tube 106surrounding the optical fibers 108 to supports and prevents the opticalfibers 108 from moving excessively within the inner tube 106. The fillermaterial 114 has sufficient viscosity to resist the shear forces appliedto it as a result of the weight of the optical fiber 108 when disposedin a vertical well installation at elevated temperatures, therebysupporting the optical fibers 108 without subjecting the fibers to thestrain of their weight. The filler material 114 has an operatingtemperature range of about 10 to about 200 degrees Celsius. However, thecable 100 may be utilized over a wider temperature range.

The filler material 114 is also configured to allow the optical fibers108 to relax and straighten with respect to the inner tube 106 due todifferences in the coefficients of thermal expansion between the opticalfiber 108 and the inner tube 106 and during spooling, deployment and useof the cable 100. The filler material 114 also prevents chaffing of thecoatings on the optical fibers 108 as a result of bending action duringinstallation and vibration of the cable 100. The filler material 114also serves as cushion the optical fiber 108 against the surface of theinner tube 106 to avoid microbend losses across cable bends. Suitablefiller 114 materials include conventional thixotropic gels or greasecompounds commonly used in the fiber optic cable industry for waterblocking, filling and lubrication of optical fiber cables. Optionally,the filler material 114 may be omitted.

To further reduce the effects of hydrogen on the optical fibers 108, thefiller material 114 may optionally include or be impregnated with ahydrogen absorbing/scavenging material 116, such as palladium ortantalum, and the like. In one embodiment, the hydrogenabsorbing/scavenging material 116 is a vanadium-titanium wire coatedwith palladium. Alternatively, the inner tube 106 may be coated with ahydrogen absorbing/scavenging material below the barrier coating 110 oron the interior surface 118 of the inner tube 106, or such a hydrogenabsorbing/scavenging material may be impregnated into the tube material,or any combination of the above.

The optical fibers 108 are selected to provide reliable transmission ofoptical signals through the cable 100 disposed in a down hole gas or oilwell application. Suitable optical fibers 108 include low defect, puresilica core/depressed clad fiber. Alternatively, suitable optical fibers108 include germanium doped single mode fiber or other optical fibersuitable for use in a high temperature environment. The optical fibers108 disposed within the inner tube 106 may be comprised of the same typeor of different types of materials. Although the invention is describedherein as using three optical fibers 108 within the inner tube 106, itcontemplated that one or more fibers 108 may be used. The total numberof fibers 108 and the diameter of the inner tube 106 are selected toprovide sufficient space to prevent microbending of the optical fibers106 during handing and deployment of the cable 100.

As the fiber optic cable 100 has an operating temperature ranging atleast between about 10 to about 200 degrees Celsius, a greater length ofoptical fibers 108 are disposed per unit length of inner tube 106 toaccount for the different coefficient of thermal expansion (CTE)represented by the optical fibers 108 and the inner tube 106. The innertube diameter is configured to accept an excess length of “serpentineover-stuff” of optical fiber 108 within the inner tube 106. In oneembodiment, the excess length of optical fiber 108 may be achieved byinserting the fiber 108 while the inner tube 106 is at an elevatedtemperature, for example, during laser welding of the inner tube 106.The temperature of the inner tube 106 is controlled such that itapproximates the anticipated maximum of normal operating temperature ofthe final installation. This process will lead to an excess length offiber 108 of up to 2.0 percent or more within the inner tube 106 coolingof the inner tube.

The FIMT core 102 is surrounded by the outer tube 104 that is configuredto provide a gap 120 therebetween. The gap 120 is filled with air orother non-structural material and provides sufficient isolation betweenthe outer tube 104 and FIMT core 102 to prevent the various layers ofthe FIMT core 102 from excessively contacting the outer tube 104 andbecoming damaged. As the FIMT core 102 and outer tube 104 are notretained relative one another, the serpentine orientation of the FIMTcore 102 within the outer tube 104 (shown in FIG. 2) results inintermittent contact points 202 therebetween. The intermittent contactpoints 202 retain the inner tube 106 relative to the outer tube 104,thus creating enough friction to prevent the inner tube 106 from movingwithin the outer tube 104 and damaging the coatings applied to theexterior of the inner tube 106.

Returning to FIG. 1, the outer tube 104 is manufactured of a corrosionresistant material that easily diffuses hydrogen. The outer tube 104 maybe manufactured of the same material of the inner tube 106 and may befabricated with or without a coating of a low hydrogen permeabilitycoating or hydrogen scavenging material. Examples of outer tubematerials include suitable corrosion resistant metal alloys such as, butnot limited to, 304 stainless steel, 316 stainless steel, INCONEL® 625and INCOLOY® 825, among others.

In one embodiment, the outer tube 104 is seam welded over the FIMT core106. The weld seam 120 of the outer tube 104 may be fabricated using aTIG welding process, a laser welding process, or any other suitableprocess for joining the outer tube 104 over the FIMT core 102.

After welding, the outer tube 104 is drawn down over the FIMT core 102to minimize the gap 120. The gap 120 ensures that the outer tube 104 isnot mechanically fixed to the FIMT core 102, thereby preventingthermally induced motion or strain during use at elevated temperaturesand/or over temperature cycling, which may damage the barrier and/orouter coatings 110, 112 if the outer tube 104 were to slide over theinner tube 106.

Alternatively, the outer tube 104 may be rolled or drown down againstthe FIMT core 102, where care is taken not to extrude or stretch theFIMT core 102 such that the excess length of the fibers 108 within theFIMT core 102 is not appreciably shortened. In embodiments where theouter tube 104 and the FIMT core 102 are in substantially continuouscontact, the inner and outer tubes 106, 104 may be fabricated from thesame material to minimize differences in thermal expansion, therebyprotecting the coating applied to the exterior of the inner tube 104.

An initial diameter of the outer tube 104 should be selected withsufficient space as not to damage the FIMT core 102 during welding. Theouter tube 104 may be drawn down to a final diameter after welding. Inone embodiment, the outer tube 104 has a final diameter of less thanabout 3/16 inch to less than about ¼ inch and has a wall thickness inthe range of about 0.7 to about 1.2 mm. Other outer tube diameters arecontemplated and may be selected to provide intermittent mechanicalcontact between the inner tube 106 and the outer tube 104 to preventrelative movement therebetween.

To further protect the cable 100 during handling and installation, aprotective jacket 122 of a high strength, protective material may beapplied over the outer tube 104. For example, a jacket 122 ofethylene-chlorotrifluoroethylene (ECTFE) may be applied over the outertube 104 to aid in the handling and deployment of the cable 100. In oneembodiment, the jacket 122 may have a non-circular cross-section, forexample, ellipsoid or irregular, or polygonal, such as rectangular. Theprotective jacket 122 may be comprised of other materials, such asfluoroethylenepropylene (FEP), polyvinylidene fluoride (PVDF),polyvinylchloride (PVC), HALAR®, TEFLON®, fluoropolymer, or othersuitable material.

As the diameter of the outer tube 104 and optional protective jacket 122result in a cable 100 that is much smaller than conventional designs,more cable 100 may be stored on a spool for transport. For example, acable 100 having a diameter of about ⅛ inch may have a length of about80,000 feet stored on a single spool, thereby allowing multiple sensingsystems to be fabricated from a single length of cable without splicing.Furthermore, the reduced diameter of the cable 100 allows for more roomwithin the well head and well bore, thereby allowing more cables (orother equipment) to be disposed within the well. Moreover, as the cable100 is lighter and has a tighter bending radius than conventionaldesigns, the cable 100 is easier to handle and less expensive to ship,while additionally easier to deploy efficiently down the well. Forexample, conventional quarter inch diameter cables typically have abending radius of about 4 inches, while an embodiment of the cable 100having an eighth inch diameter has a bending radius of less than 3inches, and in another embodiment, to about 2 inches.

FIG. 3A a cross sectional view of another embodiment of a fiber opticcable 300 suitable for use in down hole oil and gas well applications.The cable 300 is substantially similar in construction to the cable 100described above, having an FIMT core 306 disposed within a protectiveouter tube 104.

The FIMT 306 comprises an inner metal tube 302 having a polymer shell304 surrounding one or more optical fibers 108. The inner tube 302 isfabricated similar to the metal embodiment of the inner tube 302described above, while the polymer shell 304 may be applied to theexterior of the inner tube 302 by extruding, spraying, dipping or othercoating method. The polymer shell 304 may be fabricated from, but is notlimited to fluoropolymers, ethylene-chlorotrifluoroethylene,fluoroethylenepropylene, polyvinylidene fluoride, polyvinylchoride,HALAR®, TEFLON® and TEFZEL®, among others. Although the polymer shell304 is illustrated as a circular ring disposed concentrically over theinner tube 302, it is contemplated that the polymer shell 304 may takeother geometric forms, such as polygonal, ellipsoid or irregular shapes.

An optional plated barrier coating (not shown) similar to the coating110 described above, may be disposed on at least one of the inner orouter surfaces of at least one of the inner tube 302 or polymer shell304. In one embodiment, a protective outer coating (also not shown)similar to the outer coating 112 described above, is disposed over thebarrier coating 110. The outer coating 112 is a protective layer ofhard, scratch resistant material, such as nickel or a polymer such aspolyamide, among others, that substantially prevents the barrier coating110 from damage from contact with the outer tube 104.

The optical fibers 108 are selected to provide reliable transmission ofoptical signals through the cable 300 disposed in a down hole gas or oilwell application. Although the invention is described herein as usingthree optical fibers 108 within the inner tube 302, it contemplated thatone or more fibers 108 may be used. The optical fibers 108 may bedisposed in filler material 114 that substantially fills the void spaceswithin the inner tube 302 surrounding the optical fibers 108. The fillermaterial 114 may optionally be impregnated with a hydrogenabsorbing/scavenging material 116, such as palladium or tantalum, andthe like.

The outer tube 104 is configured to intermittently contact the FIMT core306 while substantially maintain a gap 120 as described above. Theintermittent contact between the inner tube 302 and FIMT core 306prevents the FIMT core 306 from moving within the outer tube 104 whileadvantageously minimizing the outer diameter of the cable 300 ascompared to conventional designs.

FIG. 3B depicts a cross sectional view of another embodiment of a fiberoptic cable 330 suitable for use in down hole oil and gas wellapplications. The cable 330 is substantially similar in construction tothe cable 300 described above, having an FIMT core 336 disposed within aprotective outer tube 104, except that the FIMT core 336 includes aplurality of fins 332.

In one embodiment, the FIMT core 336 includes an inner metal tube 302having a polymer shell 334 disposed thereover. The fins 332 extendoutwardly from the polymer shell 334. The fins 332 are typicallyunitarily formed with the shell 334 during an extrusion process, but mayalternatively be coupled to the shell 334 through other fabricationprocesses. Ends 338 of the fins 332 generally extend from the shell 334a distance configured to allow a gap 340 to be defined between the ends338 and the wall of the outer tube 104. The gap 340 allows the FIMT core336 to be disposed within the outer tube 104 in a serpentine orientation(similar to as depicted in FIG. 2), thereby allowing intermittentcontact between the FIMT core 336 and the outer tube 104 thatsubstantially secures the core 336 and outer tube 104 relative to oneanother.

Alternatively, as depicted in FIG. 3C, the outer tube 104 may be sizedor drawn down to contact the fins 332 of the FIMT core 336, thusmechanically coupling the FIMT core 336 to the outer tube 104. In thisembodiment, a gap 120 remains defined between the shell 334 and outertube 104 to substantially protect the FIMT core 336 and any coatingsdisposed thereon, while the mechanical engagement of the tube 104 andfins 332 prevent movement of the core 336 within the tube 104. Moreover,the space defined between the fins 332 provides spacing between the FIMTcore 336 and the outer tube 104 to prevent damage of the FIMT core 336during welding. Additionally, the fins 332 may be slightly comprisedduring the reduction in diameter of the outer tube 104 so that the FIMTcore 336 is not stretched or extruded in a manner that substantiallyremoves the excess length of fiber within the FIMT core 336.

FIG. 3D depicts a cross sectional view of another embodiment of a fiberoptic cable 350 suitable for use in down hole oil and gas wellapplications. The cable 350 is substantially similar in construction tothe cable 330 described above, having an fiber in tube (FIT) core 356disposed within a protective outer tube 104, except that the FIT core356 includes a plurality of fins 352 extending from a polymer inner tube354 that surrounds at least one optical fiber 108 without an interveningmetal tube.

The fins 352 are unitarily formed with the polymer inner tube 354 duringan extrusion process, but may alternatively be coupled to the inner tube354 through other fabrication processes. During fabrication, the opticalfiber 108 is disposed in the polymer inner tube 354 while the tube 354is in an expanded state, for example, immediately after the polymerinner tube 354 is extruded or after heating the tube. As the polymertube 354 cools and shrinks, the length of optical fiber 108 per unitlength of polymer tube 354 increases, thereby allowing enough opticalfiber 108 to be disposed within the polymer tube 354 to ensure minimalstress upon the optical fiber 108 after the polymer tube 354 hasexpanded when subjected to the hot environments within the well.

Ends 358 of the fins 352 generally extend from the polymer inner tube354 a distance configured to allow a gap to be defined between the ends358 and the wall of the outer tube 104 or to contact the outer wall 104as shown. In either embodiment, a gap 120 remains defined between thepolymer inner tube 354 and outer tube 104 to substantially protect theFIT core 356 and any coatings disposed thereon.

FIG. 3E depicts a cross sectional view of another embodiment of a fiberoptic cable 360 suitable for use in down hole oil and gas wellapplications. The cable 360 is substantially similar in construction tothe cable 350 described above, having an FIT core 366 disposed within aprotective outer tube 104, except that the FIT core 366 includes apolymer inner tube 364 without fins that surrounds at least one opticalfiber 108, and without an intervening metal tube.

The polymer inner tube 364 has a polygonal form, such as a triangle orpolygon (a square is shown in the embodiment depicted in FIG. 3E).However, it is contemplated that the polymer inner tube 364 may takeother geometric forms, such as polygonal, ellipsoid, circular orirregular shapes, where the polymer inner tube 364 has a differentgeometric shape than the inner diameter of the outer tube 104.

In the embodiment depicted in FIG. 3E, the polymer inner tube 364includes corners 368 that generally extend from the polymer inner tube364 a distance configured to allow a gap to be defined between thecorners 368 and the wall of the outer tube 104 or to contact the outerwall 104 as shown. In either embodiment, a gap 120 remains definedbetween the polymer inner tube 364 and outer tube 104 to substantiallyprotect the FIT core 366 and any coatings disposed thereon.

FIG. 4 depicts another embodiment of a cross sectional view of anotherembodiment of a fiber optic cable 400 suitable for use in down hole oiland gas well applications. The cable 400 is substantially similar inconstruction to the cables described above, except that the cable 400includes an expanded polymer spacer 402 that applies a force against anouter tube 104 and an FIMT core 102 that bound the spacer 402.

The polymer spacer 402 may be a foamed polymer, such as urethane orpolypropylene. In one embodiment, the polymer spacer 402 may be injectedand foamed between the outer tube 104 and the FIMT core 102 after theouter tube 104 has been welded. In another embodiment, the polymerspacer 402 may be disposed over the FIMT core 102 and compressed duringa diameter reducing step applied to the outer tube 104 after thewelding. In yet another embodiment, the polymer spacer 402 may beapplied to the exterior of the FIMT core 102, and activated to expandbetween the outer tube 104 and the FIMT core 102 after welding. Forexample, the polymer spacer 402 may be heated by passing the cable 400through an induction coil, where the heat generated by the inductioncoil causes the polymer spacer 402 to expand and fill the interstitialspace between the outer tube 104 and the FIMT core 102. As the polymerspacer 402 is biased against both the outer tube 104 and the FIMT core102, any well fluids that may breach the outer tube 104 is preventedfrom traveling along the length of the cable 400 between the outer tube104 and the FIMT core 102.

FIGS. 5–6 are a flow diagram and simplified schematic of one embodimentof a method 500 for fabricating the optic cable 330. The reader isencouraged to refer to FIGS. 5–6 simultaneously.

The method 500 begins at step 502 by extruding a polymer tube 602through a die 620 around at least one or more optical fibers 604. Theoptical fibers 604 may optionally be sheathed in a seam welded metaltube as described with reference to FIG. 1, and as described thepreviously incorporated in U.S. Pat. No. 6,404,961. As the polymer tube602 is formed, the one or more optical fibers 604 are deployed from afirst conduit or needle 612 extending through the die 620 into the tube602 to a point downstream from the extruder 606 where the polymercomprising the tube 602 has sufficiently cooled to prevent sticking ofthe fibers 604 to the tube wall at step 504. The one or more opticalfibers 604 are disposed in the tube 602 at a rate slightly greater thanthe rate of tube formation to ensure a greater length of optical fiber604 per unit length of polymer tube 602.

At an optional step 506, a filler material 608 may be injected into theinterior of the polymer tube 602 to fill the void spaces surrounding theoptical fibers 604. The filler material 608 is injected from a secondconduit or needle 610 extending through the die 620 of the polymer tube602 to a suitable distance beyond the extruder to minimize any reactionbetween the cooling polymer tube 602 and the filler material 608. Thefiller material 608 may optionally be intermixed with a hydrogenabsorbing/scavenging material.

At an optional step 508, the polymer tube 602 may be coated with abarrier material 614. The barrier material may be applied by plating,passing the tube 602 through a bath, spraying and the like. In oneembodiment, the barrier material 614 is plated on the polymer tube 602by passing the tube through one or more plating baths 618.

At an optional step 510, a protective outer sleeve 624 is formed aroundthe polymer tube 602. The outer sleeve 624 may include seam welding ametal strip 626 to form the sleeve 624 around the polymer tube 602. Theprotective outer sleeve 624 may also include a polymer jacket 628applied over the sleeve 624. The polymer jacket 628 may be formed byspraying or immersing the sleeve 624 in a polymer bath after welding. Ifa protective outer sleeve 624 is disposed over the polymer tube 602, themetal sleeve 624 may be drawn down into continuous contact with thepolymer tube 602 at step 512.

Thus, a fiber optic cable suitable for use in harsh environments such asdown hole oil and gas well applications has been provided. The noveloptic cable has unique construction that advantageously minimizesfabrication costs. Moreover, as the novel optic cable has a reduceddiameter that allows greater spooled lengths of cable facilitates moreefficient utilization as compared to conventional cable designs, therebyminimizing the cost of optical sensing systems that utilize optic cablesin down hole oil field applications.

Although the invention has been described and illustrated with respectto exemplary embodiments thereof, the foregoing and various otheradditions and omissions may be made therein and thereto withoutdeparting from the spirit and scope of the present invention.

1. An optic cable suitable for down hole oil field applications, comprising: an inner tube comprising a polymer sheath disposed over a metal tube; one or more optical fibers disposed in the inner tube; and a corrosion resistant metal outer tube disposed over the inner tube and making intermittent contact therewith, wherein the intermittent contact mechanically retains the inner tube relative to the outer tube due to frictional contact.
 2. The optic cable of claim 1, wherein an outer diameter of the inner tube has a different geometrical shape than an inner diameter the outer tube.
 3. The optic cable of claim 1 further comprising: a filler material disposed between the one or more optical fibers and the inner tube.
 4. The optic cable of claim 3, wherein the filler material disposed between the one or more optical fibers and the inner tube further comprises: a viscous material; and at least one of a hydrogen absorbing or hydrogen scavenging material intermixed in the viscous material.
 5. The optic cable of claim 1 further comprising: a low hydrogen permeable coating disposed on the inner tube.
 6. The optic cable of claim 5, wherein the low hydrogen permeable coating further comprises: a nickel seed layer disposed on the inner tube; a layer of low hydrogen permeable material disposed on the seed layer; and a nickel layer disposed over the low hydrogen permeable material layer.
 7. The optic cable of claim 5, wherein the low hydrogen permeable coating further comprises at least one of tin, gold or carbon.
 8. The optic cable of claim 5 further comprising: a polymer layer disposed on the low hydrogen permeable coating.
 9. The optic cable of claim 1, wherein the polymer sheath further comprises a plurality of fins extending outwardly therefrom.
 10. The optic cable of claim 1, wherein the outer tube has a diameter less than about 3/16 inch.
 11. The optic fiber of claim 1 further comprising a radially compressed polymer disposed between the inner tube and the outer tube.
 12. An optic cable suitable for down hole oil field applications, comprising: a metal outer tube; a polymer tube disposed within the outer tube and having a plurality of fins extending outwardly therefrom and mechanically coupling with the metal outer tube to substantially prevent movement of the polymer tube within the metal outer tube; and one or more optical fibers disposed in the polymer tube, wherein the cable includes a greater length of optical fibers per unit length of polymer tube.
 13. The optic cable of claim 12 further comprising: a filler material disposed between the one or more optical fibers and the polymer tube.
 14. The optic cable of claim 13, wherein the filler material disposed between the one or more optical fibers and the inner tube further comprises: a viscous material; and at least one of a hydrogen absorbing or hydrogen scavenging material intermixed in the viscous material.
 15. The optic cable of claim 12 further comprising: a low hydrogen permeable coating disposed on the polymer tube.
 16. The optic cable of claim 15, wherein the low hydrogen permeable coating further comprises: a nickel seed layer disposed on the inner tube; a layer of low hydrogen permeable material disposed on the seed layer; and a nickel layer disposed over the low hydrogen permeable material layer.
 17. The optic cable of claim 15, wherein the low hydrogen permeable coating further comprises at least one of tin, gold or carbon.
 18. The optic cable of claim 12 further comprising: a seam welded corrosion resistant metal tube disposed between the polymer tube and the one or more optical fibers.
 19. The optic cable of claim 18, wherein the metal tube further comprises: a low hydrogen permeable coating disposed thereon.
 20. The optic cable of claim 12, wherein the fins extend form the polymer tube to a diameter less than about 3/16 inch.
 21. An optic cable suitable for down hole oil field applications, comprising: a metal outer tube; a polymer tube disposed within the outer tube and having a different shape than an inner diameter of the metal outer tube, wherein a perimeter of the polymer tube lying in a cross sectional plane of the polymer tube intermittently contacts the inner diameter of the metal outer tube to mechanically engage the metal outer tube and secure the polymer tube and the outer tube relative to one another; one or more optical fibers disposed in the polymer tube, wherein the cable includes a greater length of optical fibers per unit length of polymer tube; and a viscous filler material disposed between the one or more optical fibers and the inner tube, wherein a hydrogen absorbing material is intermixed in the filler material. 